First your sectioning has got to be small and to perfection. Don't take to big a section. With the 45 I was always taught to place your elbow into your waist line. That assure you have the correct angle. work from the bottom up, using the perimeter as your guide. If it is a wedge cut I usually begin to do a 90 degree at the occipital bone.
With a 90 degree cut again take small sections, use your base line as your guide, and like you said it is the same length that follows the curve of the head. On the band area I usually over direct forward, as to not cut to short. You can always go back and cut more, YOU CAN"T PUT IT BACK! unless you do extensions.
With a 180 there are several variations of this depending on how much layer they want. If your shagging the hair, use your bang area as your guide, if it is longer layers, I usually council the client as to where she wants the shortest layer to be. I then hake the hair from the crown pull it straight up and then lay it against the head in the area where the shortest layer is going to be. Marking it with my finger , that would be your guide. USE SMALL SECTIONS!!!!!!! I can't elaborate that enough. even if it slow your speed down section section section.
with angeles around the face I slide cut with shears or a sharp razor.
Hope this helps.
Clear your mind, your hind end will follow